Balancing Hydronic Systems 

in Multifamily Buildings

Russell Ruch, Peter Ludwig, and 

Tessa Maurer

Partnership for Advanced Residential Retrofit

July 2014
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iii

Balancing Hydronic Systems in Multifamily Buildings

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

Russell Ruch, Peter Ludwig, and Tessa Maurer

Partnership for Advanced Residential Retrofit

322 S Green St., Suite 300

Chicago, IL 60607

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40346-04

July 2014
iv

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v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

1 Introduction........................................................................................................................................... 1

1.1 Problem Statement...............................................................................................................1

1.2 Relevance to Building America’s Goals..............................................................................2

1.3 Research Questions..............................................................................................................2

2 Literature Review.................................................................................................................................. 3

2.1 Background..........................................................................................................................3

2.2 Recent Research...................................................................................................................9

3 Experiment .......................................................................................................................................... 11

3.1 Recruitment........................................................................................................................11

3.2 Data Collection ..................................................................................................................11

3.2.1 Temperature ...........................................................................................................11

3.2.2 Gas Usage ..............................................................................................................12

3.2.3 Equipment..............................................................................................................12

3.2.4 Method of Analysis................................................................................................12

3.3 Building A..........................................................................................................................13

3.3.1 Background............................................................................................................13

3.3.2 Data Findings.........................................................................................................16

3.3.3 Analysis..................................................................................................................19

3.4 Building B..........................................................................................................................22

3.4.1 Background............................................................................................................22

3.4.2 Data Findings.........................................................................................................24

3.4.3 Analysis..................................................................................................................27

4 Discussion........................................................................................................................................... 30

4.1 Imbalance...........................................................................................................................30

4.2 Improvement......................................................................................................................30

5 Conclusion .......................................................................................................................................... 32

References................................................................................................................................................. 33

Appendix A: Sample Tenant Letter ......................................................................................................... 35

Appendix B: Outside Temperatures at Buildings A and B ................................................................... 36
vi

List of Figures

Figure 1. Diverter tees ................................................................................................................................ 4

Figure 2. Two-pipe direct return................................................................................................................ 5

Figure 3. Two-pipe reverse return............................................................................................................. 5

Figure 4. Primary/secondary piping.......................................................................................................... 6

Figure 5. Radiator (top left); convector (top right); finned-tube (bottom)............................................. 7

Figure 6. Baseboard radiator (left); radiant flooring (right).................................................................... 8

Figure 7. Fan coils (left); unit heater (right).............................................................................................. 8

Figure 8. Photograph of Building A: northeast corner (left); northwest corner (right) ..................... 13

Figure 9. Aerial view of Building A; note position of boiler in southeast corner............................... 14

Figure 10. Circulator added to west loop at Building A ........................................................................ 15

Figure 11. Building A in-unit and outdoor temperatures, pre- and post-retrofit ................................ 18

Figure 12. Two weeks of readings at unit 5651-1: pre-retrofit (left); post-retrofit (right)................... 19

Figure 13. Lowest unit temperature at Building A versus outside temperature ................................ 20

Figure 14. Difference between highest and coldest unit temperatures at 30-minute intervals ........ 21

Figure 15. Building B: note location of boiler room .............................................................................. 23

Figure 16. Primary circulator at Building B............................................................................................ 23

Figure 17. Temperatures collected at Building B .................................................................................. 25

Figure 18. Temperatures in first floor units of Building B .................................................................... 26

Figure 19. Temperatures in second floor units of Building B .............................................................. 26

Figure 20. Temperature spreads at both buildings ............................................................................... 28

Figure 21. Comparison of outside temperatures recorded at Building A and Building B ................ 36

Unless otherwise noted, all figures were created by the Partnership for Advanced Residential 

Retrofit.

List of Tables

Table 1. Buildings Included in the Study................................................................................................ 11

Table 2. Logger Equipment...................................................................................................................... 12

Table 3. Summary Statistics for Unit Temperatures (°F) in Building A ............................................... 19

Table 4. Measures of Building Spread, Before and After Retrofit, at Building A ............................... 21

Table 5. Boiler Gas Usage in Pre- and Post-Retrofit Periods at Building A........................................ 22

Table 6. Summary Statistics for Unit Temperatures (°F) in Building B ............................................... 27

Table 7. Temperature Deviation Between Recorded and Set Point Temperatures (°F) in Building B29

Unless otherwise noted, all tables were created by the Partnership for Advanced Residential 

Retrofit.
vii

Definitions

ARIES Advanced Residential Integrated Energy Solutions Collaborative

ASHRAE American Society of Heating, Refrigerating and Air Conditioning

Engineers

Btu British Thermal Unit 

DOE U.S. Department of Energy

EMS Energy Management System

HDD Heating Degree Day

NREL National Renewable Energy Laboratory

PARR Partnership for Advanced Residential Retrofit
viii

Executive Summary

More than 50% of units in colder climate region multifamily buildings in the United States are 

heated by steam or hot water systems (RECS 2009).1 These buildings often suffer from 

imbalance: some apartments are too hot and some are too cold. In hydronic systems, distribution 

issues may be caused by undersized piping, improperly adjusted balancing valves, inefficient 

water temperature and flow levels, and owner/occupant interaction with the boilers, distribution,

and controls. The effects of imbalance include tenant discomfort, higher energy use intensity,

inefficient building operation, and decreased equipment longevity. These issues can affect a 

building’s financial viability as apartments become more expensive to heat, no-heat calls 

accumulate, and maintenance staff and heating equipment work harder and less efficiently in 

their efforts to provide enough heat to all units to meet code requirements and tenant 

expectations. This paper explores the causes for heat imbalance in these systems and discusses 

potential upgrades and measures to address them, providing guidance to contractors, auditors, 

and building owners on best practices to improve tenant comfort and lower operating costs.

The research was conducted by The Partnership for Advanced Residential Retrofit in conjunction 

with Elevate Energy.2 The team drew upon existing knowledge of retrofits for optimizing 

distribution in typical multifamily hydronic systems, with the aim of identifying common 

situations and solutions, then conducted case studies on two Chicago-area buildings with known 

balancing issues to quantify the extent of temperature imbalance. At one of these buildings the 

team installed a booster pump on a loop to an under-heated wing of the building and tested the 

effect of this retrofit on imbalance. 

This study found that the spread of unit air temperatures in a multifamily hydronic building can 

reach as high as 48°F (56°–104°F), particularly if windows are opened or tenants use intermittent 

supplemental heating sources like oven ranges. This extreme disparity was even greater than that 

reported by the Advanced Residential Integrated Energy Solutions Collaborative, which found a 

spread in one building of 37.9°F (Dentz et al. 2013). The literature review found different types 

of retrofits necessary for different types of hydronic systems. Increasing flow to an underheated 

zone in the test building was found to be an effective method of improving an extreme imbalance 

issue. Average temperature spread at the building as a result of this retrofit decreased from 

22.1°F to 15.5°F.

 1 Of the 4.3 million housing units in buildings with two or more units in the “Very Cold/Cold” U.S. climate region, 

2.2 million are heated by a steam or hot water system.

2 Elevate Energy is nonprofit organization that “designs and implements efficiency programs that lower costs, 

protect the environment, and ensure the benefits of energy efficiency reach those who need them the most” (Elevate 

Energy 2014a).
1

1 Introduction

1.1 Problem Statement

Most older Chicago-area multifamily buildings are heated by centrally metered steam or 

hydronic (hot water) systems. Hydronic systems offer many advantages over steam, including 

the possibility for greater energy efficiency and more control over temperature in different areas 

of the building. Depending on the piping configuration, these systems can be zoned for each unit, 

giving tenants some control over their temperatures although the heating plant is centrally 

configured.

However, as with steam systems, these buildings can fall out of balance. Poor maintenance, 

improper renovations, or design flaws such as undersized piping can cause a hydronic system to 

heat units unevenly. Balancing the building—ensuring that the distribution system consistently 

delivers the correct rate of heat transfer to each apartment—can bring multiple benefits to 

owners, operations and maintenance staff, and tenants.

Chief among these benefits is improved tenant comfort and heating fuel efficiency. Overheated 

tenants tend to open windows to reach their desired temperature, resulting in greater space heat 

loss and increased heating fuel usage. Underheated tenants may resort to inefficient, expensive,

and sometimes dangerous secondary heating sources such as space heaters and oven ranges.

(Caleffi 2012; Deutsche Bank 2012)

Balancing systems can alleviate damage due to moisture as well. Cold air can cause shrinkage 

cracks in wood and drywall surfaces, slow drying of wetted surfaces, condensation on windows, 

and growth of mold and mildew. Air temperatures that are too high can increase air leakage 

through the building envelope and reduce interior humidity (Caleffi 2012).

Lastly, lower heating fuel costs generate operating savings that can increase cash flow and the 

bottom line while freeing up resources for other necessary building repairs. Enhanced tenant 

comfort may stabilize occupancy, thus lowering the costs of finding new tenants for owners, and 

reducing the amount of time spent by maintenance staff dealing with low-heat calls (Elevate 

2014b). 

Multifamily building owners have much to gain from improving the distribution of their 

hydronic systems. However, balancing is not a measure commonly employed by building owners 

and contractors. As Choi et al. (2012) discuss, older heating systems are often plagued by poor 

maintenance of the system and inadequate or inappropriate upgrades to parts of systems that 

compromise the system’s designed functionality and efficiency. Lacking capital funds, 

knowledge of available technologies, appropriate contractor solutions, and understanding of the 

benefits and definition of system balancing are the primary causes for misinvestment in older 

multifamily buildings. System balancing may be among the most cost-effective efficiency 

opportunities available in such buildings, though the associated comfort improvements, utility 

cost savings, and contractor methodologies are rarely well documented in the marketplace.
2

With these barriers and benefits in mind, this project seeks to:

• Survey existing knowledge about balancing hydronic buildings.

• Quantify the extent of imbalance in two test buildings.

• Measure the improvement in imbalance from retrofitting one of these buildings.

• Analyze the process of optimizing hydronic distribution systems in multifamily 

buildings.

By taking practical, cost-effective steps to optimizing their hydronic distribution systems, 

multifamily building owners can reduce costs and improve tenant comfort. This study examines 

some issues surrounding the optimization of hydronic systems, with the ultimate goal of 

providing building owners and contractors with the resources and information to make 

financially sensible and long-lasting investments in energy efficiency and tenant comfort.

1.2 Relevance to Building America’s Goals

The goal of the Building America program is to develop and prove cost-effective strategies for 

reducing energy use in buildings by up to 50%, while also seeking to identify technologies that 

lead to higher comfort, improved indoor air quality, and moisture-resistant buildings (Building 

America 2014).

This research project investigates temperature imbalance in multifamily buildings in order to 

identify retrofits that reduce energy waste from overheating and improve the comfort of tenants, 

in a colder humid continental climate, where heating is the focus of residential energy use. 

In addition to the potential for heating fuel and electricity savings, distribution optimization 

brings with it a number of other benefits in line with Building America’s goals. These include 

improved air quality, higher comfort levels throughout the building, and reduced risk of building 

deterioration that may be caused by improper moisture and/or temperature levels. 

Part of the motivation behind this project is to increase the visibility of hydronic distribution 

improvements as viable energy efficiency measures for multifamily building owners, as well as 

to streamline the identification and implementation of these measures to make them as cost

effective and transparent as possible. These goals naturally increase the scalability of the 

measures and could serve the larger aim of turning them into more widely practiced retrofit 

techniques. 

1.3 Research Questions

• How much can the temperature vary within and across units in a hot water-heated 

building?

• What are the most cost-effective and lasting improvements for hot water heating 

systems?

• Are certain balancing approaches best suited to specific building types?
3

2 Literature Review

In the first phase of this project, the Partnership for Advanced Residential Retrofit (PARR)

conducted background research and prepared a survey of existing knowledge and research 

related to multifamily building hydronic heating and balancing. Sources for this survey included 

Building America and ASHRAE publications, engineering studies, trade journals, hydronics 

design guides, seminars, and consultations with an expert hydronic heating contractor and 

Elevate Energy’s senior engineer.

2.1 Background

The relative balance of a hydronic heating system is primarily dependent on the alignment of its 

component parts, including boilers, piping configuration, circulators, air eliminators, and heat 

emitters. Briefly, this section describes how a hydronic heating system functions and some of the 

many potential issues that can lead to significant imbalance.

2.1.1 Overview of Hydronic Heating Systems

Though it has existed in some form or another for hundreds of years, hydronic heating became 

prevalent in residential buildings in the United States around the 1940s as a more efficient 

alternative to steam. In a hydronic system, hot water flows from a central boiler to heat emitters

located throughout the building, where heat from the water is transferred to the surrounding air

via highly conductive metal. The water loses its heat and cools, flowing out from the heat 

emitters and back to the boiler where it is heated again. The exact path of water depends on the 

piping configuration of the system, discussed in Section 2.1.3. 

Modern hydronic systems have a variety of components designed to enhance the performance of 

the system, including the circulator, meant to facilitate the flow of water throughout the building

and the air eliminator, which removes microbubbles from the fluid. Hydronic systems may be 

zoned in order to control or isolate water flow in certain areas of a building. Thermostats can be 

installed in each of these zones to more or less control the air temperature of the zone.

2.1.2 Circulators

In their earliest iterations, hydronic systems were designed to rely only on the lower density of 

hot water than cold water to create circulation through a closed piping system. Relatively lighter 

hot water flowed upward from the boiler to the heat emitters as colder, heavier water flowed 

downward. These “gravity hot water” systems relied solely on gravity to circulate water

throughout the building. As hydronic systems became more sophisticated, circulators were 

introduced in order to minimize friction losses and provide more efficient heat delivery. 

Circulators are sized according to the flow rate and head loss in a given heating system. Flow 

rate is determined by the heating load of the system and reflected in the diameter of the piping, 

while head loss is calculated as the pressure drop seen across the system as water flows through 

it. As a circulator cannot move more water than is permitted by the pipe size, it must be sized to 

provide the necessary head pressure at the given flow rate (Holohan 2011). 

Traditional or “single-speed” circulators are designed to provide flow at a constant rate and are 

sized for maximum demand. In recent years, engineers have developed variable speed circulators 

which have the ability to step down flow rate, thereby improving performance in systems with 
4

multiple zones and reducing unnecessary electricity use. These circulators use variable frequency 

drives, which adjust the flow rate of water in the system in response to changes in demand as 

control valves open or close. When a valve closes, the variable frequency drive reduces the speed 

of the motor, thereby adjusting the flow rate to match the demand in the system. This ensures 

relatively constant pressure throughout the system and lowers average pump speeds, resulting in 

reduced electricity use (Vastyan 2012).

2.1.3 Piping Configurations

Common hydronic piping configurations include the following (Hoger 2012; Holohan 2011):

• Series piping: Flow passes from the heat source, through each heat emitter and back to 

the heat source. As water passes through the system its temperature decreases, so heat 

emitters must be sized accordingly. While this is the simplest type of system to install and 

uses minimal piping, this configuration cannot be zoned and balancing issues are difficult 

to resolve.

• Diverter tee: Somewhat similar to series loops, diverter tee distribution systems use a 

fitting called a diverter tee to route some of the flow in the main circuit through a branch 

path with heat emitters (see Figure 1). Valves control flow through each branch, 

providing zone-by-zone comfort control when water is flowing through the main circuit, 

but temperature still decreases as water passes through the system. Diverter tee systems 

have higher installation and operation costs but also marginally more zoning control than

series piping. Balancing issues are much the same as with series piping.

Figure 1. Diverter tees

• Two-pipe direct return: In a two-pipe system, heat emitters are piped in parallel rather 

than in series, so each heat emitter receives water at approximately the same temperature. 

Zoning is much easier in a two-pipe system, as flow through each crossover can be 

independently controlled using either an on/off zone valve or a modulating flow control 

valve (see Figure 2). However, such systems also require significantly more piping than 

series piping, increasing installation costs. In addition, overall piping path length to each 

heat emitter varies based on its distance from the boiler, so flow rate diminishes for the 

furthest emitters. To correct for this problem and maintain a balanced system, balancing 

valves are installed in each crossover to allow flow rates to be adjusted according to the 

required heat output. These adjustments are best done by using preplanned calculations

rather than manual configuration by trial-and-error (Holohan 2011).
5

Figure 2. Two-pipe direct return

• Two-pipe reverse return: Again, heat emitters are piped in parallel, but return and 

supply piping are sized so that the overall piping path length for each heat emitter is 

identical. Consequently, the flow rate and temperature of the water entering each heat 

emitter are roughly the same, and balancing is much easier than in direct return systems.

Reverse return systems, however, require even more piping than two-pipe direct returns. 

Moreover, though these systems are in theory “self-balancing,” in practice, balancing 

valves are still required, as the flow resistance through each crossover will not necessarily 

be the same. Figure 3 shows the layout of a two-pipe reverse return system; the image has 

been modified to include balance valves.

Figure 3. Two-pipe reverse return
6

• Primary/secondary: A primary/secondary configuration consists of a primary loop that

delivers heated water to one or more secondary circuits. Each secondary circuit has its 

own circulator and is connected to the primary loop with pairs of closely spaced tees. The 

minimal spacing allows each circulator in the system to operate without impacting any of 

the others, thereby separating flow between the primary and secondary circuits. This 

configuration is ideal for radiant systems and systems with large temperature drops

because it causes cooler water leaving the secondary loops to mix with the still-warm 

water in the primary loop. Water returning to the boiler is therefore warmer than it would 

be if it was returned directly from the secondary loops (Joyce 2005). Figure 4 shows an 

example of a primary/secondary piping system. As the diagram indicates, it is possible to 

have multiple secondary loops with different heat loads, including an indirect load such 

as a domestic hot water heater.

Figure 4. Primary/secondary piping

2.1.4 Air Elimination

Hydronic systems are designed to be airless, closed loops. The presence of air molecules in the 

piping can compromise the longevity and performance of the system. Air molecules limit heat 

transfer by preventing heat-containing water molecules from coming into contact with the metal 

in heat emitters, reduce the efficiency of the circulator by jamming flow-check valves, create 

noise and contribute to balancing problems, and can potentially clog the distribution system by 

reacting with metals and creating rust (Holohan 2011).

Thus, air elimination is an important function of the modern hydronic system. Broadly speaking, 

air elimination occurs at two stages. After initial installation of the system, and on an annual 

basis as the system is drained and refilled between heating seasons, a technician purges air using 

bleed valves at the radiators. Air is also automatically removed during the normal operation of 

the system by the air eliminator. Since hot water has less capacity than cold water to hold air in 
7

solution, heating water causes air molecules to fall out of solution and create microbubbles inside 

the piping. The air eliminator takes advantage of the buoyancy of air to capture and expel 

microbubbles floating on the top of the water (ClimateMaster 2011; Holohan 2011). 

2.1.5 Heat Emitters

Common types of heat emitters in multifamily buildings include radiators, convectors, finnedtube, baseboard, radiant flooring, fan coils, air handlers, and unit heaters. This section briefly 

explains how each of these emitters functions.

• Radiators: Hot water flows through a series of metal fins, typically made from cast iron, 

and releases heat into the room through air convection and radiation. 

• Convectors: Similar to radiators, convectors rely on air convection to distribute heat into 

the room, but unlike radiators, they are enclosed by a sheet metal cabinet with openings 

at the bottom and top.

• Finned-tube: Very similar to convectors, finned-tube radiators differ in their use of 

metal tubing bonded to metal fins rather than the simple metal fins found in convectors.

Figure 5. Radiator (top left); convector (top right); finned-tube (bottom)

• Baseboards: Baseboard radiators may be radiative, radiative-convective, or finnedtubed. They are designed to be installed along the baseboard of a wall and thus are 

typically less conspicuous than other heat emitters. Radiative baseboards are made of 

aluminum and have no openings to induce convection. Radiative-convective baseboards 

are made of cast iron or steel and function much like convectors. Finned-tube baseboards 

are based on the same design as finned-tube units. Building owners must be mindful to 

replace or repair baseboards which have become damaged by impact, as they are 
8

relatively fragile in comparison to other emitter types. Damaged baseboard radiators can 

seriously inhibit a system’s ability to provide the necessary amount of heat to a given

unit. 

• Radiant flooring: Radiant flooring relies principally on radiative tubing, which is laid 

out at regular intervals under a floor. Hot water flows through the tubing and heats the 

room from the bottom up. Radiant flooring is most effective when covered by ceramic 

tiles, which are good heat conductors.

Figure 6. Baseboard radiator (left); radiant flooring (right)

• Fan coils: Fan coils consist simply of a fan and heating coils, which act as a heat 

exchanger. Heat is delivered to the fan coil by means of a hydronic system; the heat 

exchanger then transfers that heat load to the air, which is circulated throughout the room 

by the fan.

• Air handlers: Air handlers work much the same way as fan coils, but are typically much 

larger and attached to duct work, which may deliver heat to multiple parts of a building 

rather than just a single room.

Figure 7. Fan coils (left); unit heater (right)

• Unit heaters: Unit heaters work in much the same way as fan coils except that the 

components are contained in a single unit that, unlike most fan coils or air handlers, are

designed to be exposed to the area they are heating. They offer easy and inexpensive

installation for areas that require additional heating.
9

2.2 Recent Research

The most relevant current research on balancing strategies in multifamily hydronic systems is 

found in two studies conducted by the Advanced Residential Integrated Energy Solutions 

(ARIES) Collaborative, another Building America team. 

Dentz et al. (2014) examined three hydronically heated low-rise multifamily buildings with 

above average energy usage. The project measured energy usage before and after replacing 

modulating aquastats with boiler controls that provided more precise control.3 In one of the three

buildings studied by Dentz et al., the new controller averaged indoor temperatures from sensors 

placed in each unit. In the other two, the new controller supported nighttime setback abilities but 

not indoor temperature averaging. More sophisticated mixing valves were also added to the first 

building. The change in energy usage was examined based on a regression analysis comparing 

pre- and post-retrofit heating gas use, which showed a 10.1% to 30.9% drop in heating energy 

usage. The largest reduction was in the building with indoor temperature sensors, where the 

boiler runtime was reduced by 28%. Moreover, nearly all the savings were realized at night when 

the temperature cutoff was 5°F lower than during the day, indicating that a properly adjusted 

outdoor reset control could be enough to generate similar savings. Simple payback for all three 

buildings collectively was calculated to be about 3 years, indicating that hydronic heating 

retrofits can be cost effective. Finally, the study also concluded that a 5°F nighttime setback in 

the supply water temperature did not have any significant impact.

In a separate study, Dentz et al. (2013) collected existing archival data from energy management 

systems (EMSs) in four hydronically heated buildings, in addition to 14 steam buildings, in order 

to examine the presence and magnitude of overheating and the role of EMSs in regulating indoor 

temperatures. The study suggested that overheating is a pervasive problem in hydronic buildings 

and that the average temperature in units of hydronic buildings was above the 70°F cutoff for 

overheating whether the EMSs were on or not. Moreover, the spread of temperatures in each 

hydronic building was significant, 14.5°–37.9°F4 when the EMSs were off, and 24.5°–34°F

when EMSs were on. (Data when the EMSs were on were collected at only three of the four 

hydronic buildings). As implied by these data, the EMSs did not reliably lower the average 

temperature, nor did they narrow the temperature range in the hydronic buildings. In fact, in two 

of the three hydronic buildings with EMSs, the temperature range increased when the system 

was turned on and in one, the average temperature rose. No correlation was found between inunit temperatures and floor height. In summary, the study suggested that overheating is a 

significant and widespread problem in hydronic buildings and that such buildings suffer from 

uneven distribution. EMSs were found to have a limited ability to control temperature and 

distribution problems.

These studies indicate that overheating and temperature imbalance can be very significant issues 

in hydronic buildings and that the efficiency improvements associated with sophisticated control 

systems such as EMSs and those with averaging temperature sensors are limited. These results 

are not surprising, as EMSs are not designed to address heating imbalances. They are incapable 

 3 A report of preliminary results was released after monitoring the retrofitted buildings for the 2011–2012 heating 

season. The findings detailed here are from a draft version of the project’s final report, which includes observations 

from the 2012–2013 heating season. The final report had not yet been peer reviewed or published at the time of this 

writing.

4 Page 11, Table 3. A temperature range of 62.1°–100°F was recorded in Building 13. 
10

of sending more heat to cold apartments or less heat to warm apartments; thus, it is to be 

expected that they had no effect on temperature range.

Research presented below adds to the conversation about imbalance in hydronic systems by 

documenting serious temperature imbalances at two buildings, analyzing the factors which 

contributed to the imbalance, and evaluating a booster pump retrofit at a building with 

undersized piping. Uneven building temperatures can lead to undesired tenant responses such as 

intermittent supplemental heating or window opening, exacerbating the imbalance and impeding

the efficient use of temperature averaging controls. In these cases, increasing flow rate to an 

underheated zone may be a necessary first step before installing temperature averaging controls.
11

3 Experiment

3.1 Recruitment

In the second component of this study, PARR recruited hydronic buildings in the Elevate 

Buildings program at which to implement and study heat balancing retrofits. The following 

criteria were used to select the two buildings examined in this study:

• Suffered from unbalanced heating, as indicated by tenant complaints or sensor data.

• Owner willing to participate in the research study. Owners would potentially receive 

installed retrofit measures at no cost, and would provide access and coordination to 

Elevate staff and the contractor.

• No other upgrades to the heating system or envelope planned for the research period.

Two buildings were selected. At one building, the owner was willing to participate in logging 

and data collection but did not choose to implement retrofit measures. This building was 

included in the study (1) as a second case study of a building known to suffer from imbalance

and a source of additional high-resolution temperature data, as well as (2) as a control in 

temperature analysis for the building that was retrofitted, as it was located nearby and 

experienced nearly identical weather.

After selecting two buildings that met these criteria, the team recruited a Chicago-based heating 

contractor who specializes in hydronic heat. Working in tandem with Elevate engineers and 

construction managers, the contractor inspected the heating systems at these buildings, 

diagnosing the likely causes of imbalance in each. Based on the findings the team determined 

these buildings would be good candidates and enrolled them in the study. Table 1 summarizes 

their characteristics. Building “A” is located in Chicago and Building “B” is located in a nearby 

suburb. 

Table 1. Buildings Included in the Study

Heat Emitters Zoning Age Stories Units Size

Building A Baseboard Wing 1920 3 33 39,500 ft2

Building B Radiant flooring Unit 1961 2 10 6,750 ft2

Once the managers at both buildings agreed to enroll in the study, the contractor prepared a 

detailed scope of work for retrofitting the test building. This scope of work was approved by 

Elevate engineers and signed in a contract by the building manager, the contractor, and the 

PARR team on October 1, 2013.

3.2 Data Collection

3.2.1 Temperature

In order to record the balance of the distribution systems over the course of the sample period, 

the team installed temperature loggers in both buildings. These were installed according to 

protocols drafted by the National Renewable Energy Laboratory (NREL) for Building America 

(NREL 2013). Outdoor loggers were mounted on exterior walls in positions where they would be 
12

shaded 100% of the time. Indoor loggers were configured for 15-minute readings and installed, 

one per unit, according to the following specifications:

• On an interior wall

• Between 4 ft and 5 ft from the floor

• Not above or within 4 ft horizontally of any heating or cooling appliance

• Away from direct sunlight

• Not in close proximity to any devices that may consume a relatively large amount of 

electricity (e.g., television, aquarium, large appliances)

• In a location mutually agreed upon with the occupant

• Using damage-free hanging strips.

To ensure uncomplicated data retrieval and to minimize noise, tenants were notified in advance 

of data logger installation and given a letter explaining the project (a sample of which can be 

seen in Appendix A).

3.2.2 Gas Usage

To evaluate the impact of the retrofit on heating gas usage, the team had a natural gas submeter 

installed at Building A and piped in to measure gas usage by the boilers. Since the existing 

central gas meter at the building measured usage by the domestic hot water heaters in addition to 

the boilers, it would not be possible to know the change in usage by the boilers alone. By 

recording gas usage by the boilers during pre- and post-retrofit periods and weather-normalizing 

the data, the researchers were able to evaluate whether the retrofit increased or decreased the 

amount of energy used for central heating. 

3.2.3 Equipment

Table 2 summarizes the equipment used to record temperatures and boiler gas usage.

Table 2. Logger Equipment

Measurement Model Description

Outdoor Air Temperature HOBO U23 Pro v2 Outdoor temperature/

relative humidity logger

Indoor Air Temperature HOBO U10-003 Indoor temperature/

relative humidity logger

Boiler Gas Usage 3,500 CFH ElsterAmerican rotary gas meter Natural gas meter

3.2.4 Method of Analysis

A number of metrics were used to assess the extent, duration, and extremes of temperature 

imbalance in both buildings. Time series plots of temperature were prepared for each unit. 

Summary statistics of each unit’s temperature history were also calculated, including the 

minimum, maximum, mean, median, and standard deviation. These measures quantified the 

underheating, overheating, and temperature range in each unit. In Building A, a comparison of 
13

these metrics before and after retrofit enabled an assessment of whether and to what extent the 

retrofit improved imbalance in each unit. 

Both buildings were also evaluated according to their temperature spreads; that is, the difference 

between highest and lowest unit temperature at coincident time intervals. Since the loggers were 

not perfectly synchronous, each logger’s 15 minute readings were averaged into regular 30-

minute bins (e.g., 1:00–1:30) in order to directly compare temperature readings among units. 

Differentials between the highest and lowest temperature readings in the building for each 30-

minute period were then calculated. The measured width of temperature spread was plotted as a 

time series to show the change in imbalance over time. At Building A, this data series was used 

to assess whether the retrofit changed the percentage of time the temperature spread was wider 

than 30°F (i.e., extremely imbalanced) or less than 15°F (i.e., relatively balanced).

Unusually cold weather during the sample period made for a study of how each heating system 

performed under extremely challenging conditions. As Building B was subject to similar 

temperatures as Building A, it was possible to compare the performance of their respective

heating systems in terms of underheating, overheating, and temperature spreads. However, this

was not a perfect comparison given various differences in distribution systems, construction

type, size, and pre-retrofit heat slope between the two buildings. 

3.3 Building A

3.3.1 Background

Building A (Figure 8) was built in 1920 in an “S” shape typical of Chicago multifamily buildings 

of that era. Originally outfitted for a steam boiler, the building was converted in the late 1990s to 

a centrally metered hydronic system with baseboard radiators and multiple sequenced boilers.5

This new system was piped for only two zones, west and east, and the distribution piping was 

undersized. This resulted in chronic underheating issues in the north and west units, which were 

located furthest away from the boiler room (Figure 9).

Figure 8. Photograph of Building A: northeast corner (left); northwest corner (right)

 5 Central space heating and domestic hot water heating are master metered and paid by the owner. In-unit electricity 

and natural gas usage (i.e., oven ranges) is individually metered and paid by tenant.
14

Figure 9. Aerial view of Building A; note position of boiler in southeast corner

In 2011, equipment age and the premature failure of two boilers due to improper primary piping

led the owners to install a new hybrid mid-efficiency/high-efficiency boiler system. This system 

consisted of two Laars Mighty Therm boilers (85% TE) and two Laars Neotherm boilers (95% 

TE), all rated at 750 MBH each. These were configured in a primary/secondary piping 

arrangement and controlled by a Heat-Timer Mini-MOD-CNC condensing/non-condensing 

control.6 This hybrid system was designed to maximize efficiency gains from condensing during

shoulder season months while minimizing the up-front installation cost. The existing two 5-hp

system pumps were reused, configured to alternate every 96 hours by a Tekmar pump sequencer. 

For primary boiler controls, a TRS RC2100 was installed with eight wireless temperature sensors 

placed in units around the building.7 This system controlled boiler firing according to the

average recorded temperature and allowed maintenance to monitor temperatures around the 

building. Around the same time, the building’s roof cavity was air sealed and insulated in an 

effort to slow heat loss and further reduce natural gas usage. Following these retrofits, the 

building’s centrally metered natural gas energy use intensity declined by 23%, averaging 88

kBtu/ft2/year for the 2 years leading up to this study.8,9

 6 This outdoor reset multiple boiler heating control changes the lead boiler group to condensing when the target 

outdoor temperature is below a specified switching set point.

7 Temperature sensors were located in the following units: 5657-1N, 5653-1, 52-3, 5645-3N, 62-2, 5647-3N, 

5645-1N, 5657-3N. 8 Energy use intensity was calculated by taking the utility bills for the central natural gas account, separating usage 

into heating load and baseload based on usage in the summer months, and weather-normalizing according to 30-year 

average heating degree days (HDDs).
15

In July 2013, building management expressed concerns that despite the new temperature controls

the building still had underheating issues in the north and, especially, west units. They noted that 

uneven heat distribution caused many tenants to open windows or supplement heat with their 

oven ranges. These temperature-altering actions interfered with the averaging controls to the 

extent that the cut-in set point had to be set at 79°F in order for the boiler to consistently heat all 

units to at least 65°F. The high set point resulted in severe overheating of some units, causing 

significant tenant discomfort and unnecessary gas usage. Ultimately building management 

decided to bypass the new temperature averaging controls altogether and use only the outdoor 

reset as the primary firing control. 

In August 2013, PARR’s heating contractor submitted a proposal for a retrofit to address the 

imbalance issue. Since replacing the undersized piping would be prohibitively expensive, he 

recommended adding a booster pump to increase flow to the west loop (Figure 10). This retrofit 

was expected to address underheating of the furthest units, while allowing maintenance to return 

to using the temperature averaging controls, ideally at a lower set point temperature. Building 

management agreed to this proposal and signed a contract on October 1, 2013.

Figure 10. Circulator added to west loop at Building A

In October 2013 the contractor drained the system and installed isolating valves on the west loop 

so the pump could be installed in the middle of the heating season (for data comparison 

purposes) with minimal disruption. Eighteen data loggers were installed throughout the building, 

comprising a sample of about 50% of units. In November 2013 the contractor installed a boiler 

gas meter. 

In December 2013, the team collected preliminary pre-retrofit data from the data loggers and 

noted boiler gas usage. An unrelated flooding issue in the basement earlier in the year required

unexpected minor repairs on two of the boilers, which were performed by another contractor on 

January 8. In order to have comparable pre- and post-retrofit data that would be unaffected by 

this repair, retrofit was delayed 4 weeks and the pump was installed on February 6. However, for 

reasons unknown to management, the maintenance staff turned off the pump at some point 

before the contractor returned on February 11. During this visit the contractor was told some 

 9 At the start of the study this building performed better than 76% of other hot water heated buildings built before 

1960 in the Elevate Buildings program.
16

units were suddenly not getting heat, and he advised maintenance to bleed problem units and 

check for proper circulation. By February 13 the no-heat issues had been resolved and the pump 

was running again.

Thus, in the following data analysis “pre-retrofit” refers to the period of January 9–February 5 

and “post-retrofit” refers to the period of February 14–March 30. Outside temperature during 

these periods was relatively similar: average HDD per day in the pre-retrofit period were 36, as 

opposed to 44 in the post-retrofit period.

Data collection continued after retrofit through the end of March. Thirteen in-unit data loggers10

and the outside logger were retrieved on April 1. At this time loggers were tested at room 

temperature to ensure their sensors were still functioning properly. One logger (5657-2N) was

excluded from the study because of faulty readings. 

Two loggers showed periods of unusually low readings. These were in the following units.

• 5655-3: For reasons unknown to building management, temperature dropped suddenly 

from 73.8°F to 48.9°F at 10:00 p.m. on January 14. Unusually low readings continued 

until around 12:30 p.m. on January 18, when the temperature returned to the 70+°F 

range. Temperature began to dip again into the 50s°F on January 20 at 1:00 p.m. and did 

not return to normal range (64°–82°F) until 5:00 p.m. on January 24. Due to this 

anomalous pattern of behavior, readings for this unit between January 14 and January 24

were not used in this study. 

• 52-1: This unit was vacant from January 13 to March 1. Temperature dropped 

significantly, dipping as low as 50°F. Management believes a window may have been left 

open during this period. Because the unit included so many low temperature outliers 

during this time, readings for this period were not used in the analysis for this study. 

3.3.2 Data Findings

In-unit temperatures collected in the pre-retrofit period were uneven and erratic, with a wide 

spread of temperatures across units. These data confirmed a temperature imbalance across the 

building and suggested that tenants were altering their unit temperatures by opening windows 

and using supplemental heating such as oven ranges. Figure 11 plots the temperatures in each 

unit over the course of the sample period. Before the retrofit, temperatures were 55°–115°F, 

rising above 90°F in units 52-2, 5645-1N, and 5651-1. After retrofit, temperatures were 59°–

104°F and peaked above 90°F in units 52-1 and 52-2 (3 times), 5651-1 (once), and 5657-1N. 

As an illustration of the extreme temperature fluctuations recorded by the loggers, Figure 12

compares readings in Unit 5651-1 over the course of 2 different weeks. Located in the west wing 

of the building, this unit experienced very high temperature peaks in the pre-retrofit period. 

Temperatures rose above 100°F during 144 of the 7,943 readings recorded after January 9, and 

all of these 100+°F readings occurred before the pump installation on February 6. Note that this 

particular logger had been installed near the kitchen but followed NREL protocol in being more 

than 4 ft away horizontally from any heat emitting appliances. However, the extreme 

 10 Four data loggers went missing after installation.
17

temperatures and relative proximity to the kitchen suggest that the oven range was used to 

supplement heat in the unit. 

Temperature in the pre-retrofit week oscillates widely within a range of 66°–111°F, cycling one 

to three times a day. Temperature in the post-retrofit week rises and falls within a relatively 

narrower range of 66°–86°F, and only rises above 81°F during a single cycle. Outdoor 

temperatures during these weeks were comparable.
18

Figure 11. Building A in-unit and outdoor temperatures, pre- and post-retrofit

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

1/9 1/14 1/19 1/24 1/29 2/3 2/8 2/13 2/18 2/23 2/28 3/5 3/10 3/15 3/20 3/25 3/30

Temperature (°F)

52-1

52-2

62-2

62-3

5645-1N

5645-2N

5645-3N

5649-3

5651-1

5655-3

5657-1N

outside

Retrofit
19

Figure 12. Two weeks of readings at unit 5651-1: pre-retrofit (left); post-retrofit (right)

3.3.3 Analysis

3.3.3.1 Summary of Unit Temperatures

A number of metrics were calculated in order to quantify the changes in unit temperatures before 

and after retrofit. Table 3 gives summary statistics for each unit.

Table 3. Summary Statistics for Unit Temperatures (°F) in Building A

Min Max Mean Median SD

Unit Pre Post Pre Post Pre Post Pre Post Pre Post

52-1 58.1 65.2 79.0 103.7 66.2 79.2 67.1 76.7 4.1 7.2

52-2 64.7 64.8 98.7 93.9 78.3 76.6 75.7 76.9 7.6 3.8

62-2 66.2 67.6 82.0 85.0 74.0 76.5 74.1 76.5 2.5 2.3

62-3 64.7 71.7 82.7 87.0 71.3 80.6 70.1 80.9 4.1 1.6

5645-1N 53.4 58.5 98.5 89.8 73.3 69.8 71.9 70.0 9.3 3.9

5645-2N 56.9 65.3 77.4 79.3 67.8 71.6 68.2 71.5 4.7 2.9

5645-3N 62.9 71.0 91.2 89.2 76.1 78.8 76.0 78.8 4.9 2.2

5649-3 72.9 72.5 88.7 86.1 80.4 82.0 80.4 82.2 2.6 1.5

5651-1 53.3 63.1 114.0 94.4 77.5 73.0 74.2 73.4 11.5 3.4

5655-3 65.5 64.8 79.7 82.2 73.0 76.9 74.1 77.6 3.6 3.0

5657-1N 55.0 62.9 86.5 105.4 72.2 75.2 72.7 73.9 5.9 5.4

Averaged 61.2 66.1 88.9 90.5 73.6 76.4 73.1 76.2 5.5 3.4

The results indicate that unde-heating became less significant as a result of the retrofit. Minimum 

recorded temperature rose in nine of the 11 units and dropped by less than 1°F in the other two 

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

Temperature (°F)

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

Temperature (°F)
20

units. On average, unit minimum temperature rose by 4.9°F after the retrofit, with the greatest 

improvement in the west wing unit 5651-1 (9.8°F). Figure 13 plots the minimum recorded unit 

temperature in Building A at each 30-minute interval over the course of the study period.

Figure 13. Lowest unit temperature at Building A versus outside temperature

Meanwhile, standard deviations dropped in 10 of the 11 units, representing a reduction in the 

magnitude of the temperature swings. This was especially true in units 5651-1 and 5645-1N

(located respectively on the west and north sides), where this metric improved by 8.1°F and 

5.4°F, respectively. Mean standard deviation in all units decreased from 5.5°F to 3.4°F.

While temperature spreads did narrow and underheating was less common after retrofit, median 

temperatures also rose slightly in eight of the 11 units. On average, median temperatures 

increased by an average 3.1°F. (Excluding outlier unit 52-1, which experienced a wide range of 

temperatures between 70 and 104°F in the post-retrofit period, the average median temperature 

rise was 1.5°F.) Every unit had median temperatures in the post-retrofit period above 70°F.

Seven of the 11 units had median temperatures above 75°F after retrofit, as opposed to only three

of the 11 units before retrofit, suggesting that overheating had become a problem and the outdoor 

reset control was set too high. However, given the narrower temperature spread across units 

(shown in the next section) and the higher minimum unit temperatures, these data suggest that 

maintenance may be able to lower the cut-in set point next year and reduce heating fuel usage.

3.3.3.2 Analysis of Temperature Imbalance Across Units

Mean differentials for the pre-retrofit and post-retrofit periods showed an improvement in 

temperature spread from 22.1°F to 15.5°F. This measure of temperature spread is plotted over 

time in Figure 14. Pre- and post-retrofit averages are shown in green and red.

-10

0

10

20

30

40

50

60

70

80

Temperature (°F )

Pre-Retrofit Post-Retrofit

Lowest unit temperature
21

Figure 14. Difference between highest and coldest unit temperatures at 30-minute intervals

As Figure 14 visually depicts, not only did the temperature spread across units generally improve 

after retrofit, but the incidence of temperature differences likely caused by supplemental heating 

also significantly decreased. This is shown by the relatively few “spikes” in temperature spread

after retrofit. Though some tenants likely continued to supplement the baseboard heat, they seem 

to have been compelled to do so less often.

Table 4 compares the temperature spreads across the building before and after retrofit using 15°F

and 30°F as markers. These measures indicate that temperature spreads tended to be less extreme 

after retrofit.

Table 4. Measures of Building Spread, Before and After Retrofit, at Building A

Pre-Retrofit Post-Retrofit

Proportion of half-hour intervals 

in which building spread >30°F 17.3% 1.3%

Proportion of half-hour intervals 

in which building spread <15°F 25.4% 52.4%

3.3.3.3 Gas Usage Analysis

Gas meter readings were analyzed for pre- and post-retrofit periods. Readings in cubic feet were 

converted to kBtu by a conversion factor of 1.027 kBtu to 1 ft3. Energy usage was then 

normalized using the number of HDDs recorded for that period. Table 5 summarizes the pre- and 

post-retrofit usage findings by the boilers.

Weather-normalized boiler gas usage did increase at this building as a result of the retrofit. 

However, certain potential contributing factors should be taken into account when interpreting 

this result. 

0

5

10

15

20

25

30

35

40

45

50

55

1/9 1/16 1/23 1/30 2/6 2/13 2/20 2/27 3/6 3/13 3/20 3/27

Temperature spread (°F)
22

Table 5. Boiler Gas Usage in Pre- and Post-Retrofit Periods at Building A

Boiler Gas Usage 

(ft3)

Boiler Gas Usage 

(kBtu) HDD kBtu/HDD

Pre-Retrofit 326,487 335,302 1,257 266.7

Post-Retrofit 575,185 590,715 1,476 400.2

As shown in Table 3, some units were underheated in the pre-retrofit period. Ten of the 11 units 

experienced temperatures lower than 68°F prior to retrofit, and mean temperatures in 2 units 

were below 68°F. Maintenance and ownership indicated several times before and during the 

study that some tenants would sometimes use oven ranges to supplement the heat in their units. 

The extreme and sudden air temperature spikes recorded over the course of the sample period are 

evidence of this. In the absence of sufficient, even heating from their baseboards, some tenants 

appear to have taken the heating situation into their own hands.

After retrofit, the temperature spikes decreased in frequency, suggesting that tenants used their 

oven ranges less. However, average temperatures also increased in nine of the 11 units. Since 

heat from oven ranges apparently decreased but the air temperatures did not decrease, it is likely 

that the heating load was shifted from tenants’ oven ranges onto the boiler. Oven ranges at the 

building are individually metered, and the meter installed for this study measured boiler gas 

usage only. If more of the heating load was supplied by the boiler than by oven ranges, it is 

expected that boiler gas usage would increase as a result of this retrofit. This would explain the 

rise in gas usage in Table 5. 

As has been suggested above, it is possible that next heating season maintenance may be able to 

lower the set point and/or use the temperature averaging controls as a result of the pump retrofit,

which improved the balance in the building. Doing so would likely decrease boiler gas usage, 

resulting in savings to the building owner. Shifting the heating load away from oven ranges and 

onto the central heating system would be a necessary first step before these savings are possible.

3.4 Building B

3.4.1 Background

Building B is a 10-unit, two-story, California style building constructed in 1961. Roughly 

arranged in an “L” shape with five units on each floor (Figure 15), the building uses an 

underfloor radiant heating system with individual zones for each apartment. The boiler room is 

located in a basement in the southeast corner.11

 11 Like Building A, central space heating and domestic hot water heating are master metered and paid for by the 

owner. In-unit electricity and natural gas usage (i.e., oven ranges) is metered by unit and paid for by the tenants.
23

Figure 15. Building B: note location of boiler room

At the start of this study, the original 435 MBH boiler from the 1960s was still in use. Apartment 

unit temperatures were controlled by Honeywell wall thermostats. A Tekmar indoor/outdoor 

temperature controller was set for the return temperature at 130°F and the outdoor cutout was set 

at 55°F. A Taco 121-3 pump served as the primary circulator (Figure 16). The owner had also 

installed a Grundfos UP 15-42 F booster pump on the second floor, to increase flow to the two 

furthest units. This single-speed pump was controlled by a thermostat in one of those two units, 

where the tenant had agreed not to change the thermostat setting. As at Building A, centrally 

metered natural gas energy use intensity was calculated for the building using gas bill analysis 

and weather normalization. The building averaged 66 kBtu/ft2

/year for the 2 years prior to this 

study, better than 59% of other hot water heated multifamily buildings built after 1960 in the 

Elevate Buildings program.

Figure 16. Primary circulator at Building B

During our initial conversation about the study in April 2013, the owner mentioned tenant

complaints of overheating on the first floor. This issue had persisted for years despite the zone 

valves having recently been replaced and significant tenant turnover during that period. The 
24

owner was able to rule out leaky underfloor piping or misreading thermostats as other potential 

causes. (We were able to confirm the latter in the temperature data we collected.)

In November 2013 data loggers were installed in every unit throughout the building. Preliminary 

pre-retrofit data were collected from the loggers in December 2013, and the loggers were 

retrieved in April 2014.

3.4.2 Data Findings

Temperatures at Building B were significantly less volatile and wide ranging than those at 

Building A (Figure 17). The narrower band of temperatures in building B can be attributed to 

various factors, including the separate zones for each unit, more appropriately sized piping, its 

smaller building footprint, and the overall higher level of attention and maintenance paid by the 

owner of Building B to the heating system there.
25

Figure 17. Temperatures collected at Building B

-10

-505

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

Temperature (°F)

1234567890

outside
26

In line with the owner’s concern, the first floor did regularly experience higher temperatures than

the second floor during the sample period. First floor unit readings were generally ranged 

between 76°F and 85°F (Figure 18), while on the second floor temperature tended to oscillate

between 72°F and 82°F (Figure 19). Note that second floor unit #8 read especially low because

the tenant there preferred to set the thermostat at 60°F.

Figure 18. Temperatures in first floor units of Building B

Figure 19. Temperatures in second floor units of Building B

69

71

73

75

77

79

81

83

85

87

Temperature (°F)

1

2

3

4

5

60

62

64

66

68

70

72

74

76

78

80

82

84

Temperature (°F)

6

7

8

9

10
27

3.4.3 Analysis

3.4.3.1 Summary of Unit Temperatures

Metrics were calculated to evaluate the range of unit temperatures at Building B. Table 6 gives

summary statistics by unit.

Table 6. Summary Statistics for Unit Temperatures (°F) in Building B

Unit Min Max Mean Median SD

1 72.0 88.1 80.0 79.7 1.7

2 75.5 83.6 79.2 79.2 1.1

3 75.0 82.3 79.3 79.5 1.0

4 69.6 86.6 81.3 81.6 2.6

5 73.4 85.7 80.4 80.7 2.1

6 66.2 84.1 75.1 74.6 3.3

7 70.8 77.4 73.8 73.9 1.2

8 60.4 72.0 64.8 64.7 1.8

9 70.8 83.8 78.5 78.3 2.3

10 71.0 83.6 77.9 78.1 2.1

Averaged 70.5 82.7 77.0 77.0 1.9

These figures confirm the higher first floor temperatures seen in Figure 18 and Figure 19. 

Median temperatures in the first floor units were 79.2°–81.6°F (mean 80.1°F), while the second 

floor unit medians were 64.7°–78.3°F (mean 73.9°F). These ranges indicate a moderate 

temperature disparity between floors, but they are not proof of imbalance. Since units were 

individually zoned, it is possible that first floor tenants could have had higher thermostat set

points than those on the second floor. Set points are dealt with in Section 3.4.3.3.

3.4.3.2 Comparison of Unit Temperatures Between Test Buildings

In every measure shown on Table 3, Building B performed as well as or better than Building A:

• Underheating: Minimum unit temperatures suggest that underheating was much less an 

issue here than at Building A. With the exception of unit #8, where the tenant preferred to 

keep a low set point, unit temperatures never dropped below 66.2°F. Meanwhile, even 

after retrofit minimum unit temperatures at Building A ranged from 58.5°F to 67.6°F.

• Temperature variability: Units here experienced narrower temperature bands. The 

mean unit standard deviation for Building B was 1.9°F versus 3.4°F for post-retrofit 

Building A.

• Overheating: Building B units were heated to about the same degree as Building A, as 

indicated by the average temperatures measured. Median unit temperatures topped 75°F 

in seven of 10 units here versus seven of 11 units at Building B, with averages at 77.0°F 

and 76.2°F, respectively.

Weather conditions at Building A and Building B were similar, as seen by a comparison of 

outside temperatures recorded at each, found in Appendix B.
28

Figure 20 compares the temperature spread at each building over the sample period. The 

temperature spread at Building B was clearly less volatile than that at Building A, indicating a 

more evenly heated building where intermittent supplemental heating in the units occurred less 

frequently. However, spread was not always lower than at Building A, reflecting the unit-specific 

zoning in Building B, which allowed tenants to change their unit temperatures to some extent.

Figure 20. Temperature spreads at both buildings

3.4.3.3 Consideration of Set Point Temperatures

Thermostat set points for each unit were recorded on December 19, 2013 during preliminary data

retrieval. They are shown in Table 7, along with the actual temperature readings recorded at the 

time, along with several other metrics. As it was not possible to record changes to the thermostat 

settings in real time, the assumption was made that these recorded set points remained relatively 

stable over the sample period. Taking the recorded set points as constants, we calculated 

temperature deviations in units over time as the difference between recorded and set point 

temperature. The maximum and median deviations over the sample period are given for each 

unit.

0

10

20

30

40

50

60

1/9 1/16 1/23 1/30 2/6 2/13 2/20 2/27 3/6 3/13 3/20 3/27

Temperature spread (°F)

A

B
29

Table 7. Temperature Deviation Between Recorded and Set Point Temperatures (°F) in Building B

Unit Set Point 

(12/19)

Actual 

(12/19)12

Deviation Max

(11/13 – 3/31)

Deviation Median

(11/13 – 3/31)

1 75 78.4 13.1 4.7

2 70 79.3 13.6 9.2

3 75 79.5 7.3 4.5

4 60 83.2 26.6 21.6

5 74 79.0 11.7 6.7

6 74 78.2 10.1 0.6

7 73 74.6 4.4 0.9

8 60 64.3 12.0 4.7

9 73 80.0 10.8 5.3

10 75 78.4 8.6 3.1

The disparity between set point and actual temperatures recorded on December 19 gives further

evidence of an overheating problem on the first floor. For example, in unit #4, the temperature 

measured at the time of the visit was 83°F despite the thermostat having been set to 60°F. 

(Logger data show that the temperature did not decline significantly in the hours after recording 

this set point.) This wide discrepancy between desired and actual temperature was not seen in 

second floor unit #8, on the other hand, where the thermostat was set at 60°F and actual 

temperature was 64.3°F. This suggests the tenant in the first floor unit had much less control 

over temperature in the unit than the tenant on the second floor, even though the building was 

zoned for individual unit control. Temperature in unit #4 was apparently determined more by the 

temperatures in contiguous units #3, #5, and especially, #9, than by the temperature set point of 

the thermostat. This reflects the in-floor location of the radiant piping, which could cause 

significant “bleed-through” from a second floor unit to the first floor unit below.

The deviation measures in Table 7 quantify the pattern of heating in each unit during the sample 

period. The maximum deviation shows how wide this discrepancy got during the sample period. 

The median deviation indicates the average discrepancy between the desired and actual 

temperature in the unit. Again, unit #4 experienced the highest discrepancy at a median 21.6°F. 

 12 Actual temperatures are from the data loggers, but they were also cross-checked against the thermostat 

temperature readings. For each unit these values were found to match within 1°F.
30

4 Discussion

4.1 Imbalance

Temperature data collected at the two test buildings illustrate two examples of significant 

temperature imbalances that can exist in multifamily hydronic buildings. Using high-resolution 

data, we found that one unit in Building A experienced temperatures that varied by as much as 

61°F (unit #5651-1). It is likely this wide range was the result of a tenant opening windows when 

the unit was too hot and using the oven range for heating when the unit was too cold.

This data also showed the temperature spread across units which is possible in this building type. 

At several points in the pre-retrofit period, temperatures across different units in Building A 

differed by as much as 48°F (56°F to 104°F). It is to be expected that in a 100-year-old building 

with only two zones and a boiler that is not centrally located, some units would have higher 

temperatures than others. However, the extent of the temperature disparity between units was 

likely the result of the tenant behaviors discussed above.

Interestingly, temperature spreads at Building B, which was individually zoned and of newer 

construction, were not always lower than those at Building A. During much of the post-retrofit 

period, Building A experienced spreads in the 10°–15°F range while Building B ranged between 

15°F and 20°F differences. This is most likely to be explained by the fact that one of the units in 

Building B had set the thermostat at 60°F; although wide, temperature spreads at Building B 

were fairly regular and did not have the extreme peaks experienced by Building A. Standard 

deviation in spread was only 1.9°F versus 3.4°F at post-retrofit Building A.

By other measures, Building B performed as well as or better than Building A when subjected to 

the same weather conditions. Minimum unit temperatures never dropped below 66.2°F, but postretrofit Building A saw temperatures as low as 58.5°F (though they were uncommon). Median 

unit temperatures in both buildings topped 75°F in about 70% of the units, indicating 

overheating.

4.2 Improvement

Though conditions in Building A were still less than optimal after retrofit, the addition of a 

booster pump to its west wing did significantly improve issues with underheating, imbalance, 

and extreme heating possibly caused by oven ranges. In nine of 11 units, the lowest recorded 

temperature rose after retrofit, for an average 4.9°F increase. The average building temperature 

spread decreased from 22.1°F to 15.5°F. Finally, standard deviation in unit temperatures 

decreased from 5.5°F to 3.4°F, indicating a reduction in the extreme temperature swings that 

were likely the result of window opening and supplemental heating.

This building had temperature averaging controls installed in 2011, but this retrofit proved 

ineffective in a situation where one of the wings was chronically underheated and tenants 

regularly made significant changes to the temperature in their units through methods unrelated to 

the distribution system. Faced with frequent low-heat and over-heating calls, maintenance 

decided to bypass the temperature averaging controls and revert to solely using the outdoor reset. 

The set point was set to nearly 80°F in an effort to prevent low-heating complaints, and the 

building continued to experience wide-ranging temperatures.
31

Increasing flow rate to the underheated wing served to stabilize the situation and bring the 

system into better balance. Now that this has been achieved, it is more likely that maintenance 

will have fewer low-heat and overheating calls, and they should at the very least be able to return 

to reduce the set point on the outdoor reset control and hopefully will in fact return to using the 

temperature averaging controls. Experience at this building suggests that it is necessary to get the 

flow rate right before installing efficiency retrofits such as temperature sensors to “fine-tune” the 

boiler firing. 
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5 Conclusion

This research project helped to provide an answer to a question issued by the Building America 

Expert Meeting on hydronic heating in multifamily buildings: How much can the temperature 

vary within units in a building heated by hot water? With high-resolution temperature data 

collected in line with NREL protocol from two buildings, we found that unit air temperatures in 

a poorly balanced building can range by as much as 48°F (56°F to 104°F), particularly if 

windows are opened or tenants use intermittent supplemental heating sources like oven ranges. 

This extreme result was even higher than that reported by the ARIES Collaborative, which 

reported an instantaneous spread in one building of 37.9°F.

The literature review and dissimilar temperatures seen at each building showed that different 

types of retrofits are necessary for different types of hydronic buildings. As previous PARR 

research on single-pipe steam systems showed, simple balancing retrofits such as vent 

replacement and temperature averaging controls can be cost-effective measures for saving 

energy and improving tenant comfort (Choi 2012). Hydronic systems are much more diverse, 

including a range of piping configurations, heat emitter styles, and zoning methods. Poor heating 

balance can result from undersized piping, undersized circulators, malfunctioning zone valves, 

damaged heat emitters, poor air elimination, misconfigured balancing valves, or a number of 

other mechanical issues. It can be further exacerbated by tenant behaviors such as window 

opening or oven range heating, which can dramatically and quickly change the temperature in a 

unit. 

As results from Building A indicate, adding a booster pump to an underheated zone can be one 

effective method of balancing out an extremely uneven multifamily hydronic building. After 

retrofit, the average temperature spread in Building A decreased from 22.1°F to 15.5°F. 

Experience at this building suggests that, in cases where unit temperatures are significantly 

affected by tenants opening windows or using oven ranges to regulate unit temperature, a 

temperature averaging control retrofit will likely be ineffective in improving the efficiency of the 

system. Maintenance may choose to revert to prior, relatively inefficient controls and raise the 

set points on those to very high temperatures to avoid low-heat calls in underheated units. 

Increasing flow to an underheated wing can help stabilize the situation and allow for more “finetuning” of the system with temperature averaging controls. Energy reduction from and cost

effectiveness of the retrofit in these cases is highly contingent on the extent to which tenants are 

supplying their own heat with their own sources. However, the potential improvements to tenant 

comfort, health, and safety, not to mention the reduced time and cost spent dealing with tenant 

complaints, make hydronic balancing highly beneficial to owner and tenants alike, in buildings 

facing very significant balance problems.
33

References

ASHRAE (2012). ASHRAE Handbook: HVAC Systems and Equipment, Hydronic HeatDistributing Units and Radiators.

Building America (2014). Research for the American Home. Website. Accessed 3/11/2014 

at http://energy.gov/eere/buildings/building-america-research-american-home.

Caleffi Hydronic Solutions (2012). Hydronic Fundamentals. idronics Journal of Design 

Innovation for Hydronic Professionals. Issue 12, January 2013.

Choi, J., Ludwig, P., Brand, L. (2012). Steam System Balancing and Tuning for Multifamily 

Residential Buildings in Chicagoland. Partnership for Advanced Residential Retrofit. July 2012.

ClimateMaster (2011). Essentials of Hydronics for GSHP Professionals. 

Dentz, J.; Henderson, H.; Varshney, K. (2014). “Hydronic Heating Retrofits for Low-Rise 

Multifamily Buildings.” The ARIES Collaborative.

Dentz, J.; Varshney, K.; Henderson, H. (2013). “Overheating in Hot Water- and Steam-Heated 

Multifamily Buildings.” The ARIES Collaborative. 

Deutsche Bank (2012). The Benefits of Energy Efficiency in Multifamily Affordable Housing.

Elevate Energy (2014a). “About Elevate Energy.” Accessed on 4/17/2014 

at http://www.elevateenergy.org/about/.

Elevate (2014b). Preserving Affordable Multifamily Housing through Energy Efficiency: NonEnergy Benefits of Energy Efficiency Building Improvements. January 2014.

Hoger, R., and Bailey, B. (2012). “Hydronic System Fundamentals.” Seminar at Temperature 

Equipment Corporation (TEC) Center on October 24, 

2012. http://tecstorage.com/controls/hydronic/1%20-%20Hydronic%20Fundamentals.pdf

Holohan, D (2011). Classic Hydronics: How to Get the Most From Those Older Hot-Water 

Heating Systems. 

Joyce, M. (2005). Residential Construction Academy: Plumbing. Thomson Delmar Learning.

Landry, R. (2012). Variable Speed Pumping in Condensing Boiler Systems. Webinar, October 9, 

2012. http://mncee.org/Innovation-Exchange/Resources/Variable-Speed-Pumping-inCondensing-Boiler-System/

Liao, Z.; Dexter, A.L. (2004). “The potential for energy saving in heating systems through 

improving boiler controls.” Energy and Buildings; 36(3); pp. 261-271.

NREL (2013). The Building America Indoor Temperature Study Measurement Protocol, Version 

04. Draft, February 28, 2013. Golden, CO: National Renewable Energy Laboratory.
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RECS (2009). Table HC6.6 Space Heating in U.S. Homes, by Climate Region, 2009. Residential 

Energy Consumption Survey

Vastyan, John (2012). “Variable-Speed Pumping.” Accessed 18 April 2014 at 

http://hpac.com/mag/variable-speed-pump-1212.
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Appendix A: Sample Tenant Letter13

 13 CNT Energy is the former name of Elevate Energy. The name was changed in the middle of the study.
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Appendix B: Outside Temperatures at Buildings A and B

Figure 21. Comparison of outside temperatures recorded at Building A and Building B

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Building A

Building B
DOE/GO-102014-4463 ▪ July 2014

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.

buildingamerica.gov